Impedance transformation with transistor circuits

ABSTRACT

An impedance transformation circuit utilizes two transistor circuits.

BACKGROUND

In some instances, an amplifier device can require very high inputimpedance. For example, a capacitive sensor feeding the amplifier devicemay require an impedance in the giga-ohm range. Simply inserting a verylarge resistor between an input of the amplifier device and a bias pointto generate an appropriate operating point consumes too much siliconarea to be practical.

Impedance transformations may be used to increase input impedance of theamplifier circuit. One impedance transformation technique relates tocoupling a pair of transistor circuits in a current mirror arrangementwith each transistor circuit of the pair having a different channelwidth to channel length (W/L) ratio. In some implementations, thetransistor circuit having a lower W/L ratio acts as an impedance device.In these implementations, the quotient in the W/L ratios of thetransistor circuits is proportional to the impedance transformation. Toillustrate, when the W/L ratio of a first transistor circuit isapproximately ten times larger than the W/L ratio of a second transistorcircuit, the impedance value of the second transistor circuit increasesby a factor of ten. However, circuit arrangements relying on differingW/L ratios in a pair of transistor circuits for impedance transformationare only easily achievable for impedances in the mega-ohm range. Inorder to produce impedances in the giga-ohm range for these circuitarrangements, currents on the order of picoamps may be required, makingthe impedance given by these circuit arrangements impractical.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference number in different instances in thedescription and the figures may indicate similar or identical items.

FIG. 1 is a schematic diagram of a system including a high impedancecircuit coupled between a source and an amplifier circuit.

FIG. 2 is a schematic diagram of a high impedance circuit includingnegative channel metal oxide semiconductor (NMOS) transistor circuitscoupled to a voltage source providing different overdrive voltages toeach transistor circuit.

FIG. 3 is a schematic diagram of a high impedance circuit includingpositive channel metal oxide semiconductor (PMOS) transistor circuitscoupled to a voltage source providing different overdrive voltages toeach transistor circuit.

FIG. 4 is a schematic diagram of a high impedance circuit including aseries connection between one NMOS transistor circuit of a pair of NMOStransistor circuits and one PMOS transistor circuit of a pair of PMOStransistor circuits, where a first voltage source provides differentoverdrive voltages to the NMOS transistor circuits and a second voltagesource provides different overdrive voltages to the PMOS transistorcircuits.

FIG. 5 is a schematic diagram of a high impedance circuit including aparallel connection between one NMOS transistor circuit of a pair ofNMOS transistor circuits and one PMOS transistor circuit of a pair ofPMOS transistor circuits, where a first voltage source providesdifferent overdrive voltages to the NMOS transistor circuits and asecond voltage source provides different overdrive voltages to the PMOStransistor circuits.

FIG. 6 is a schematic diagram of a high impedance circuit including NMOStransistor circuits coupled to an impedance element and a current sourceto provide different overdrive voltages to the NMOS transistor circuits.

FIG. 7 is a schematic diagram of a high impedance circuit including agate to drain coupling of NMOS transistor circuits having differentoverdrive voltages.

FIG. 8 is a flow diagram of a method of producing impedancetransformations with transistor circuits.

DETAILED DESCRIPTION

This disclosure describes using impedance transformations to provide ahigh impedance circuit at an input of an amplifier circuit using arelatively small amount of silicon area. In addition, the high impedancecircuit utilizing the impedance transformations provides good controlover the impedance value. The impedance transformations may be producedwith negative channel metal oxide semiconductor (NMOS) transistorcircuits and positive channel metal oxide semiconductor (PMOS)transistor circuits. Impedance transformation may take place due todifferences in the channel width to channel length ratios of transistorcircuit pairs.

Additionally, impedance transformation may take place due to differencesin overdrive voltages of transistor circuit pairs. An overdrive voltageis defined by a gate-source voltage of a transistor circuit minus athreshold voltage of the transistor circuit. A transistor circuit havingan overdrive voltage lower than the overdrive voltage of anothertransistor circuit may serve as an impedance device. In someimplementations, at least the transistor circuit with the loweroverdrive voltage is operating with a gate-source voltage in thesub-threshold region. A transistor circuit is operating with agate-source voltage in the sub-threshold region when the gate-sourcevoltage is less than the voltage that turns on the transistor circuit(i.e. when the overdrive voltage is a negative value). When thegate-source voltage of the transistor circuit serving as the impedancedevice is in the sub-threshold region, the impedance increasesexponentially in relation to the difference between the overdrivevoltages of a pair of transistors. For example, when the overdrivevoltage of the impedance device transistor is a specified amount lowerthan the overdrive voltage of another transistor circuit, the impedanceof the impedance device transistor may increase by a factor of ten. Inone illustration, the impedance of the impedance device transistor mayincrease by a factor of ten for every 80-90 mV difference between theoverdrive voltages of the transistor circuits. In some implementations,the differences in the overdrive voltages may be produced via the gatesof the transistor circuits. In other implementations, the differences inthe overdrive voltages may be produced via the sources of the transistorcircuits.

FIG. 1 is a schematic diagram of a system 100 to provide high impedanceat an input of an amplifier circuit. A source 102 and an amplifiercircuit 104 are connected by a source-amplifier line 106. The source 102may include one or more circuit arrangements that provide input signalsto the amplifier circuit 104. The input signals may include radiofrequency signals, audio signals, digital signals, other signalscarrying data, or a combination thereof. In some implementations, thesource 102 may be a capacitive voltage source. For example, the source102 may be a silicon microphone.

The amplifier circuit 104 may include one or more circuit arrangementsto amplify the signals received from the source 102 and provide them toa load 108. In some instances, the amplifier circuit 104 may requirevery high input impedance. For example, the amplifier circuit 104 mayrequire very high input impedance when amplifying a signal from a smallcapacitive sensor having a capacitance of a few picofarads and havinglow corner frequencies, such as a silicon microphone. The load 108 mayinclude an output device, such as an audio speaker, an analog to digitalconversion circuit, a mixer, or a combination thereof.

The system 100 also includes a high impedance circuit 110 to provide awell-defined DC voltage at a very high impedance level. The highimpedance circuit 110 may utilize impedance transformations to providethe high impedance input to the amplifier circuit 104. The highimpedance circuit 110 produces a first impedance transformation 112. Thefirst impedance transformation 112 may be produced by one or moretransistor circuit pairs having different channel width to channellength (W/L) ratios. For example, the first impedance transformation 112may be produced by a first transistor circuit having a first W/L ratiocoupled to a second transistor circuit having a second W/L ratio that isless than the first W/L ratio. In this example, the second transistorcircuit serves as an impedance device with an impedance transformationproportional to the quotient of the first W/L ratio and the second W/Lratio. The W/L ratio of a transistor circuit may be realized by a singletransistor. Alternatively, the W/L ratio of a transistor circuit may berealized by connecting a number of transistors in parallel or byconnecting a number of transistors in series. The first impedancetransformation 112 may be produced by a number of pairs of transistorcircuits coupled to each other, where the transistor circuits of eachrespective pair have differing W/L ratios. The transistor circuits mayinclude NMOS transistors and/or PMOS transistors.

The high impedance circuit 110 may also produce a second impedancetransformation 114. The second impedance transformation 114 may beproduced by providing different overdrive voltages to each of a pair oftransistor circuits coupled to each other. Different overdrive voltagesmay be produced when the difference between a gate-source voltage and athreshold voltage of a first transistor circuit is offset from thedifference between a gate-source voltage and a threshold voltage of asecond transistor circuit. In one example, a first transistor circuitand a second transistor circuit may have similar threshold voltages. Inthis example, the first transistor circuit may have a first gate-sourcevoltage and the second transistor circuit may have a second gate-sourcevoltage that is smaller than the first gate-source voltage. In this way,the first transistor circuit and the second transistor circuit havediffering overdrive voltages. In another example, the first transistorcircuit and the second transistor circuit may have similar gate-sourcevoltages, but the first transistor circuit may have a first thresholdvoltage and the second transistor circuit may a second threshold voltagethat is smaller than the first threshold voltage. Consequently, thefirst transistor circuit and the second transistor circuit havedifferent overdrive voltages.

The difference between the overdrive voltages of the first transistorcircuit and the second transistor circuit may be produced by placing thefirst transistor circuit and the second transistor circuit in variousarrangements, such that the overdrive voltage of the transistor circuitserving as the impedance device is produced by offsetting the overdrivevoltage of the other transistor circuit by a particular amount. Inparticular illustrative implementations, the first transistor circuitand the second transistor circuit are arranged in a current mirror typearrangement. In other implementations, the difference between theoverdrive voltages of the first transistor circuit and the secondtransistor circuit is produced by modifying the gate-source voltage ofthe first transistor circuit via an impedance device, such as aresistor, feeding the modified gate-source voltage into a number ofoperational amplifiers, and providing the output voltage of theoperational amplifier arrangement to the second transistor circuit.

In some implementations, the gate-source voltages of the first andsecond transistor circuits may be provided by a voltage source coupledto the first transistor circuit and coupled to the second transistorcircuit. Additionally, the respective gate-source voltages may be in thesub-threshold region of the transistor circuits. The gate-sourcevoltages may be produced through the gates of the transistor circuits,the sources of the transistor circuits, or a combination thereof. Thesecond impedance transformation 114 may be produced using a number ofpairs of transistor circuits, with the transistor circuits of eachrespective pair having different overdrive voltages. The pairs oftransistor circuits may include NMOS transistors, PMOS transistors, or acombination thereof.

By producing the first impedance transformation 112 in conjunction withthe second impedance transformation 114, the high impedance circuit 110may provide very high impedance with respect to the input of theamplifier circuit 104. Thus, the high impedance circuit 110 may providehigh input impedance to the amplifier circuit 104, while minimizing thesilicon area used to achieve the high input impedance. In addition, thecurrents provided to the high impedance circuit 110 to produce the veryhigh input impedance of the amplifier circuit 104 are large enough forthe impedance produced by the high impedance circuit 110 to be accurate.

FIG. 2 is a schematic diagram of a high impedance circuit 200 includingnegative channel metal oxide semiconductor (NMOS) transistor circuitscoupled to a voltage source providing different overdrive voltages toeach transistor circuit. The high impedance circuit 200 includes a firsttransistor circuit 202. The first transistor circuit 202 may have afirst channel width to channel length (W/L) ratio. In addition, thefirst transistor circuit 202 includes one or more NMOS transistorsrepresented by a first NMOS transistor 204. Thus, the W/L ratio of thefirst transistor circuit 202 may be realized in a single NMOS transistoror in a number of NMOS transistors. In some instances, the W/L ratio maybe realized by a number of NMOS transistors connected in parallel. Agate of the first NMOS transistor 204 is coupled to a voltage source 206and a source of the first NMOS transistor 204 is coupled to ground. Adrain of the first NMOS transistor 204 receives a reference current,I_(REF).

The voltage source 206 includes a number of components, such as animpedance element 208 and a current source 210. The impedance element208 may include one or more resistors or any arrangement of otherdevices, such as one or more capacitors, one or more transistors, one ormore inductors, or a combination thereof. Additionally, the voltagesource 206 may include an NMOS transistor, rather than the currentsource 210. In some implementations, the voltage source 206 is alsocoupled to a reference point, such as ground. Further, the voltagesource 206 is coupled to a second transistor circuit 212.

The second transistor circuit 212 may have a second W/L ratio that isless than the first W/L ratio. The W/L ratio of the second transistorcircuit 212 may be realized by one or more NMOS transistors representedby a second NMOS transistor 214. In some instances, the W/L ratio of thesecond transistor circuit 212 may be realized by a number of NMOStransistors connected in series. A gate of the second NMOS transistor214 is coupled to the voltage source 206 and a source of the second NMOStransistor 214 is coupled to ground. Further, a drain of the second NMOStransistor 214 is coupled to a source-amplifier line 216. Thesource-amplifier line 216 is coupled at an input of an amplifier device,such as the source-amplifier line 106 connecting the source 102 and theamplifier circuit 104 of FIG. 1.

A first gate-source voltage of the first NMOS transistor 204 is definedby the current I_(REF) provided to the first NMOS transistor 204 and thecurrent drawn from the first NMOS transistor 204 by the voltage source206. In this way, a well-defined gate-source voltage can be produced atthe first NMOS transistor 204. Additionally, this arrangement mayproduce other well-defined small signal parameters, such as thetransconductance of the first NMOS transistor 204. A second gate-sourcevoltage of the second NMOS transistor 214 is defined by a difference ofthe gate-source voltage of the first NMOS transistor 204 and a voltagedrop provided by the voltage source 206. The voltage drop of the voltagesource 206 may be defined by the product of the impedance value of theimpedance element 208 and the current drawn by the current source 210.Thus, the second NMOS transistor 214 can have a well-defined gate-sourcevoltage, which also results in a well-defined R_(ON), that is, theimpedance value that turns on the second NMOS transistor 214.

In the illustrative implementation shown in FIG. 2, the first transistorcircuit 202 and the second transistor circuit 212 are placed in acurrent mirror type arrangement and the high impedance circuit 200 maybe used to generate an impedance transformation by producing a firstoverdrive voltage at the first NMOS transistor 204 and a secondoverdrive voltage at the second NMOS transistor 214 that is differentfrom the first overdrive voltage. In some implementations, an overdrivevoltage of the first NMOS transistor 204 may be different from anoverdrive voltage of the second NMOS transistor 214 due to differinggate-source voltages at the first NMOS transistor 204 and the secondNMOS transistor 214, while the threshold voltages of the first NMOStransistor 204 and the second NMOS transistor 214 are approximately thesame. In other implementations, the overdrive voltage of the first NMOStransistor 204 may be different from the overdrive voltage of the secondNMOS transistor 214 due to differing threshold voltages of the firstNMOS transistor 204 and the second NMOS transistor 214, while thegate-source voltages of the first NMOS transistor 204 and the secondNMOS transistor 214 are approximately the same. In a particular example,when the overdrive voltage of the second NMOS transistor 214 is lessthan the overdrive voltage of the first NMOS transistor 204, theimpedance value of the first NMOS transistor 204 is transformed to theimpedance value of the second NMOS transistor 214 giving a very highimpedance value for the high impedance circuit 200.

In some instances, the gate-source voltage of the first NMOS transistor204, the gate-source voltage of the second NMOS transistor 214, or acombination thereof, may be in a sub-threshold region. When thegate-source voltage of the second NMOS transistor 214 is in thesub-threshold region, the impedance of the second NMOS transistor 214may increase exponentially as the overdrive voltage of the second NMOStransistor 214 decreases. The amount of increase in the impedance of thesecond NMOS transistor 214 may be based on the difference in theoverdrive voltages of the first NMOS transistor 204 and the second NMOStransistor 214.

In addition, when the W/L ratio of the second transistor circuit 212 isless than the W/L ratio of the first transistor circuit 202, the secondtransistor circuit 212 may serve as an impedance device and an impedancetransformation may take place with respect to the second transistorcircuit 212 that is proportional to the quotient of the W/L ratio of thefirst transistor circuit 202 and the W/L ratio of the second transistorcircuit 212. Thus, by producing a first impedance transformation bydifferent overdrive voltages for the NMOS transistors 204, 214, inconjunction with a second impedance transformation utilizing differentW/L ratios for the transistor circuits 202, 212, the high impedancecircuit 200 can provide a very high impedance for an amplifier device,such as the amplifier circuit 104 of FIG. 1. Further, although FIG. 2shows that the gate-source voltages of the transistors 204, 214 arerealized via the gates of the transistors 204, 214, the gate-sourcevoltages may be realized via the sources of the transistors 204, 214.

FIG. 3 is a schematic diagram of a high impedance circuit 300 includingpositive channel metal oxide semiconductor (PMOS) transistor circuitscoupled to a voltage source providing different overdrive voltages toeach transistor circuit. The high impedance circuit 300 includes a firsttransistor circuit 302. The first transistor circuit 302 may have afirst channel width to channel length (W/L) ratio. In addition, thefirst transistor circuit 302 includes one or more PMOS transistorsrepresented by a first PMOS transistor 304. Thus, the W/L ratio of thefirst transistor circuit 302 may be realized in a single PMOS transistoror in a number of PMOS transistors. In some instances, the W/L ratio maybe realized by a number of PMOS transistors connected in parallel. Agate of the first PMOS transistor 304 is coupled to a voltage source 306and a source of the first PMOS transistor 304 is coupled to a positivesupply voltage, V_(DD). A drain of the first PMOS transistor 304receives a reference current, I_(REF).

The voltage source 306 includes a number of components, such as animpedance element 308 and a current source 310. Additionally, thevoltage source 306 may include an NMOS transistor, rather than thecurrent source 310. In some implementations, the voltage source 306 isalso coupled to a reference point, such as ground. Further, the voltagesource 306 is coupled to a second transistor circuit 312.

The second transistor circuit 312 may have a second W/L ratio that isless than the first W/L ratio. The W/L ratio of the second transistorcircuit 312 may be realized by one or more PMOS transistors representedby a second PMOS transistor 314. In some instances, the W/L ratio of thesecond transistor circuit 312 may be realized by a number of PMOStransistors connected in series. A gate of the second PMOS transistor314 is coupled to the voltage source 306 and a source of the second PMOStransistor 314 is coupled to V_(DD). Further, a drain of the second PMOStransistor 314 is coupled to a source-amplifier line 316. Thesource-amplifier line 316 is coupled at an input of an amplifier device,such as the source-amplifier line 106 connecting the source 102 and theamplifier circuit 104 of FIG. 1.

A first gate-source voltage of a first PMOS transistor 304 is defined bythe current I_(REF) provided to the first PMOS transistor 304 and thecurrent drawn from the first PMOS transistor 304 by the voltage source306. In this way, a well-defined gate-source voltage can be produced atthe first PMOS transistor 304. Additionally, this arrangement mayproduce other well-defined small signal parameters, such as thetransconductance of the first PMOS transistor 304. A second gate-sourcevoltage of the second PMOS transistor 314 is defined by a difference ofthe gate-source voltage of the first PMOS transistor 304 and a voltagedrop provided by the voltage source 306. The voltage drop of the voltagesource 306 may be defined by the product of the impedance value of theimpedance element 308 and the current drawn by the current source 310.Thus, the second PMOS transistor 314 can have a well-defined gate-sourcevoltage, which also results in a well-defined R_(ON), that is, theimpedance value that turns on the second PMOS transistor 314.

In the illustrative implementation shown in FIG. 3, the first PMOStransistor 304 and the second PMOS transistor 314 are placed in acurrent mirror type arrangement and the high impedance circuit 300 maybe used to generate an impedance transformation by producing a firstoverdrive voltage at the first PMOS transistor 304 and a secondoverdrive voltage at the second PMOS transistor 314 that is differentfrom the first overdrive voltage. In some implementations, an overdrivevoltage of the first PMOS transistor 304 may be different from anoverdrive voltage of the second PMOS transistor 314 due to differinggate-source voltages at the first PMOS transistor 304 and the secondPMOS transistor 314, while the threshold voltages of the first PMOStransistor 304 and the second PMOS transistor 314 are approximately thesame. In other implementations, the overdrive voltage of the first PMOStransistor 304 may be different from the overdrive voltage of the secondPMOS transistor 314 due to differing threshold voltages of the firstPMOS transistor 304 and the second PMOS transistor 314, while thegate-source voltages of the first PMOS transistor 304 and the secondPMOS transistor 314 are approximately the same. In a particular example,when the overdrive voltage of the second PMOS transistor 314 is lessthan the overdrive voltage of the first PMOS transistor 304, theimpedance value of the first PMOS transistor 304 is transformed to theimpedance value of the second PMOS transistor 314 giving a very highimpedance value for the high impedance circuit 300.

In some instances, the gate-source voltage of the first PMOS transistor304, the gate-source voltage of the second PMOS transistor 314, or acombination thereof, may be in a sub-threshold region. When thegate-source voltage of the second PMOS transistor 314 is in thesub-threshold region, the impedance of the second PMOS transistor 314may increase exponentially as the overdrive voltage of the second PMOStransistor 314 decreases. The amount of increase in the impedance of thesecond PMOS transistor 314 may be based on the difference in theoverdrive voltages of the first PMOS transistor 304 and the second PMOStransistor 314.

In addition, when the second W/L ratio is less than the first W/L ratio,the second transistor circuit 312 may serve as an impedance device andan impedance transformation may take place with respect to the secondtransistor circuit 312 that is proportional to the quotient between thefirst W/L ratio and the second W/L ratio. Thus, by producing a firstimpedance transformation by different overdrive voltages for the PMOStransistors 304, 314, in conjunction with a second impedancetransformation utilizing different W/L ratios for the transistorcircuits 302, 312, the high impedance circuit 300 can provide a veryhigh impedance for an amplifier device, such as the amplifier circuit104 of FIG. 1. In addition, although FIG. 3 shows that the gate-sourcevoltages of the transistors 304, 314 are realized via the gates of thetransistors 304, 314, the gate-source voltages may be realized via thesources of the transistors 304, 314.

FIG. 4 is a schematic diagram of a high impedance circuit 400 includinga series connection between one NMOS transistor circuit of a pair ofNMOS transistor circuits and one PMOS transistor circuit of a pair ofPMOS transistor circuits, where a first voltage source providesdifferent overdrive voltages to the NMOS transistor circuits and asecond voltage source provides different overdrive voltages to the PMOStransistor circuits. The high impedance circuit 400 includes a firstNMOS transistor circuit 402. The first NMOS transistor circuit 402 mayhave a first NMOS channel width to channel length (W/L) ratio. Inaddition, the first NMOS transistor circuit 402 includes one or moreNMOS transistors represented by a first NMOS transistor 404. Thus, theW/L ratio of the first NMOS transistor circuit 402 may be realized in asingle NMOS transistor or in a number of NMOS transistors. In someinstances, the first NMOS W/L ratio may be realized by a number of NMOStransistors connected in parallel. A gate of the first NMOS transistor404 is coupled to a first voltage source 406 and a source of the firstNMOS transistor 404 is coupled to a line 408. A drain of the first NMOStransistor 404 receives a reference current, I_(REF1).

The first voltage source 406 includes a number of components, such as afirst impedance element 410 and a first current source 412. The firstvoltage source 406 is also coupled to the line 408. In an alternativeimplementation, the first current source 412 may be coupled to groundrather than to the line 408. Additionally, the first voltage source 406is coupled to a second NMOS transistor circuit 414. The second NMOStransistor circuit 414 may have a second NMOS W/L ratio that is lessthan the first NMOS W/L ratio. The W/L ratio of the second NMOStransistor circuit 414 may be realized by one or more NMOS transistorsrepresented by a second NMOS transistor 416. In some instances, the W/Lratio of the second NMOS transistor circuit 414 may be realized by anumber of NMOS transistors connected in series. A gate of the secondNMOS transistor 416 is coupled to the first voltage source 406 and asource of the second NMOS transistor 416 is coupled to a connectionpoint 418 on a source-amplifier line between a source and an amplifierdevice, such as the source-amplifier line 106 connecting the source 102and the amplifier circuit 104 of FIG. 1. Further, a drain of the secondNMOS transistor 416 provides a series connection to a first PMOStransistor circuit 420.

The first PMOS transistor circuit 420 may have a first PMOS channelwidth to channel length (W/L) ratio. In some implementations, the firstPMOS W/L ratio may be different from the second NMOS W/L ratio of thesecond NMOS transistor circuit 414. In addition, the first PMOStransistor circuit 420 includes one or more PMOS transistors representedby a first PMOS transistor 422. Thus, the W/L ratio of the first PMOStransistor circuit 420 may be realized in a single PMOS transistor or ina number of PMOS transistors. In some instances, the first PMOS W/Lratio may be realized by a number of PMOS transistors connected inseries. A source of the first PMOS transistor 422 is coupled to a secondvoltage source 424 and, in some implementations, a gate of the firstPMOS transistor 422 is coupled to a reference point, such as ground.

The second voltage source 424 includes a number of components, such as asecond impedance element 426 and a second current source 428. In someimplementations, the second voltage source 424 is also coupled to areference point, such as ground. Additionally, the second voltage source424 is coupled to a second PMOS transistor circuit 430. The second PMOStransistor circuit 430 may have a second PMOS W/L ratio that is greaterthan the first PMOS W/L ratio. In some implementations, the second PMOSW/L ratio may be different from the first NMOS W/L ratio of the firstNMOS transistor circuit 402. The W/L ratio of the second PMOS transistorcircuit 430 may be realized by one or more PMOS transistors representedby a second PMOS transistor 432. In some instances, the W/L ratio of thesecond PMOS transistor circuit 430 may be realized by a number of PMOStransistors connected in parallel. A source of the second PMOStransistor 432 is coupled to the second voltage source 424. In addition,the source of the second PMOS transistor 432 receives a second referencecurrent, I_(REF2). In some implementations, a drain of the second PMOStransistor 432 and a gate of the second PMOS transistor 432 are coupledto a reference point, such as ground.

A first NMOS gate-source voltage of the first NMOS transistor 404 isdefined by the current I_(REF1) provided to the first NMOS transistor404 and the current drawn from the first NMOS transistor 404 by thefirst voltage source 406. In this way, a well-defined gate-sourcevoltage can be produced at the first NMOS transistor 404. Additionally,this arrangement may produce other well-defined small signal parameters,such as the transconductance of the first NMOS transistor 404. A secondNMOS gate-source voltage of the second NMOS transistor 416 is defined bya difference of the gate-source voltage of the first NMOS transistor 404and a voltage drop provided by the voltage source 406. The voltage dropof the voltage source 406 may be defined by the product of the impedancevalue of the impedance element 410 and the current drawn by the firstcurrent source 412. Thus, the second NMOS transistor 416 can have awell-defined gate-source voltage, which also results in a well-definedR_(ON), that is, the impedance value that turns on the second NMOStransistor 416.

In the illustrative implementation shown in FIG. 4, the first NMOStransistor circuit 402 and the second NMOS transistor circuit 414 areplaced in a current mirror type arrangement and the high impedancecircuit 400 may be used to generate an impedance transformation byproducing a first NMOS overdrive voltage at the first NMOS transistor404 and a second NMOS overdrive voltage at the second NMOS transistor416 that is different from the first overdrive voltage. In someimplementations, the first NMOS overdrive voltage may be different fromthe second NMOS overdrive voltage due to differing gate-source voltagesat the first NMOS transistor 404 and the second NMOS transistor 416,while the threshold voltages of the first NMOS transistor 404 and thesecond NMOS transistor 416 are approximately the same. In otherimplementations, the first NMOS overdrive voltage may be different fromthe second NMOS overdrive voltage due to differing threshold voltages ofthe first NMOS transistor 404 and the second NMOS transistor 416, whilethe gate-source voltages of the first NMOS transistor 404 and the secondNMOS transistor 416 are approximately the same. In a particular example,when the second NMOS overdrive voltage of the second NMOS transistor 416is less than the first NMOS overdrive voltage of the first NMOStransistor 404, the impedance value of the first NMOS transistor 404 istransformed to the impedance value of the second NMOS transistor 416giving a very high impedance value for the high impedance circuit 400.

In some instances, the gate-source voltage of the first NMOS transistor404, the gate-source voltage of the second NMOS transistor 416, or acombination thereof, may be in a sub-threshold region. When thegate-source voltage of the second NMOS transistor 416 is in thesub-threshold region, the impedance of the second NMOS transistor 416may increase exponentially as the overdrive voltage of the second NMOStransistor 416 decreases. The amount of increase in the impedance of thesecond NMOS transistor 416 may be based on the difference in theoverdrive voltages of the first NMOS transistor 404 and the second NMOStransistor 416.

In addition, when the W/L ratio of the second NMOS transistor circuit414 is less than the W/L ratio of the first NMOS transistor circuit 402,the second NMOS transistor circuit 414 may serve as an impedance deviceand an impedance transformation may take place with respect to thesecond NMOS transistor circuit 414 that is proportional to the quotientof the W/L ratio of the first NMOS transistor circuit 402 and the W/Lratio of the second NMOS transistor circuit 414. Thus, by producing afirst impedance transformation by different overdrive voltages for theNMOS transistors 404, 416, in conjunction with a second impedancetransformation utilizing different W/L ratios for the transistorcircuits 402, 414, the high impedance circuit 400 can provide a veryhigh impedance for an amplifier device, such as the amplifier circuit104 of FIG. 1.

A first PMOS gate-source voltage of the first PMOS transistor 422 isdefined by a difference of the gate-source voltage of the second PMOStransistor 432 and a voltage drop provided by the second voltage source424. The voltage drop of the second voltage source 424 may be defined bythe product of the impedance value of the second impedance element 426and the current drawn by the second current source 428. Thus, the firstPMOS transistor 422 can have a well-defined gate-source voltage, whichalso results in a well-defined R_(ON), that is, the impedance value thatturns on the first PMOS transistor 422. A second PMOS gate-sourcevoltage of the second PMOS transistor 432 is defined by the currentI_(REF2) provided to the second PMOS transistor 432 and the currentdrawn from the second PMOS transistor 432 by the second voltage source424. In this way, a well-defined gate-source voltage can be produced atthe second PMOS transistor 432. Additionally, this arrangement mayproduce other well-defined small signal parameters, such as thetransconductance of the second PMOS transistor 432.

In the illustrative implementation shown in FIG. 4, the first PMOStransistor circuit 420 and the second PMOS transistor circuit 430 areplaced in a current mirror type arrangement and the high impedancecircuit 400 may be used to generate an impedance transformation byproducing a first PMOS overdrive voltage at the first PMOS transistor422 and a second PMOS overdrive voltage at the second PMOS transistor432 that is different from the first PMOS overdrive voltage. In someimplementations, the first PMOS overdrive voltage may be different fromthe second PMOS overdrive voltage due to differing gate-source voltagesat the first PMOS transistor 422 and the second PMOS transistor 432,while the threshold voltages of the first PMOS transistor 422 and thesecond PMOS transistor 432 are approximately the same. In otherimplementations, the first PMOS overdrive voltage may be different fromthe second overdrive voltage due to differing threshold voltages of thefirst PMOS transistor 422 and the second PMOS transistor 432, while thegate-source voltages of the first PMOS transistor 422 and the secondPMOS transistor 432 are approximately the same. In a particular example,when the first PMOS overdrive voltage is less than the second PMOSoverdrive voltage, the impedance value of the second PMOS transistor 432is transformed to the impedance value of the first PMOS transistor 422giving a very high impedance value for the high impedance circuit 400.

In some instances, the gate-source voltage of the first PMOS transistor422, the gate-source voltage of the second PMOS transistor 432, or acombination thereof, may be in a sub-threshold region. When thegate-source voltage of the first PMOS transistor 422 is in thesub-threshold region, the impedance of the first PMOS transistor 422 mayincrease exponentially as the overdrive voltage of the first PMOStransistor 422 decreases. The amount of increase in the impedance of thefirst PMOS transistor 422 may be based on the difference in theoverdrive voltages of the first PMOS transistor 422 and the second PMOStransistor 432.

In addition, when the W/L ratio of the second PMOS transistor circuit430 is greater than the W/L ratio of the first PMOS transistor circuit420, the first PMOS transistor circuit 420 may serve as an impedancedevice and an impedance transformation may take place with respect tothe first PMOS transistor circuit 420 that is proportional to thequotient of the W/L ratio of the second PMOS transistor circuit 430 andthe W/L ratio of the first PMOS transistor circuit 420. Thus, producinga third impedance transformation utilizing different W/L ratios for thePMOS transistor circuits 420, 430, in conjunction with producing afourth impedance transformation with different overdrive voltages forthe PMOS transistors 422, 432, contributes to the impedance provided bythe high impedance circuit 400 to the amplifier device.

In some instances, impedance transformations via the first NMOStransistor circuit 402 and the second NMOS transistor circuit 414 mayproduce non-linear behavior with respect to the impedance provided bythe high impedance circuit 400 to the amplifier device. For example, thefirst NMOS transistor circuit 402 and the second NMOS transistor circuit414 may be high ohmic with high voltage signals and low ohmic with lowvoltage signals. The impedance transformation that occurs with respectto the first PMOS transistor circuit 420 and the second PMOS transistorcircuit 430 helps to offset at least some of the non-linearity in theimpedance produced by the first NMOS transistor circuit 402 and thesecond NMOS transistor circuit 414. To illustrate, when the impedanceproduced by the first NMOS transistor circuit 402 and the second NMOStransistor circuit 414 is too high ohmic, then the impedance produced bythe first PMOS transistor circuit 420 and the second PMOS transistorcircuit 430 is low ohmic, since the impedance produced by the first PMOStransistor circuit 420 and the second PMOS transistor circuit 430 islower with high voltage signals. In this way, the first PMOS transistorcircuit 420 and the second PMOS transistor circuit 430 linearize theimpedance of the high impedance circuit 400. In addition, when theimpedance produced by the first NMOS transistor circuit 402 and thesecond NMOS transistor circuit 414 is too low ohmic, the impedanceproduced by the first PMOS transistor circuit 420 and the second PMOStransistor circuit 430 is high ohmic, thus at least partiallylinearizing the impedance of the high impedance circuit 400.

FIG. 5 is a schematic diagram of a high impedance circuit 500 includinga parallel connection between one NMOS transistor circuit of a pair ofNMOS transistor circuits and one PMOS transistor circuit of a pair ofPMOS transistor circuits, where a first voltage source providesdifferent overdrive voltages to the NMOS transistor circuits and asecond voltage source provides different overdrive voltages to the PMOStransistor circuits. The high impedance circuit 500 includes a firstNMOS transistor circuit 502. The first NMOS transistor circuit 502 mayhave a first NMOS channel width to channel length (W/L) ratio. Inaddition, the first NMOS transistor circuit 502 includes one or moreNMOS transistors represented by a first NMOS transistor 504. Thus, theW/L ratio of the first NMOS transistor circuit 502 may be realized in asingle NMOS transistor or in a number of NMOS transistors. In someinstances, the first NMOS W/L ratio may be realized by a number of NMOStransistors connected in parallel. A gate of the first NMOS transistor504 is coupled to a first voltage source 506 and a source of the firstNMOS transistor 504 is coupled to a line 508. A drain of the first NMOStransistor 504 receives a reference current, I_(REF1).

The first voltage source 506 includes a number of components, such as afirst impedance element 510 and a first current source 512. The firstvoltage source 506 is also coupled to the line 508. In an alternativeimplementation, the first current source 512 may be coupled to groundrather than to the line 508. Additionally, the first voltage source 506is coupled to a second NMOS transistor circuit 514. The second NMOStransistor circuit 514 may have a second NMOS W/L ratio that is lessthan the first NMOS W/L ratio. The W/L ratio of the second NMOStransistor circuit 514 may be realized by one or more NMOS transistorsrepresented by a second NMOS transistor 516. In some instances, the W/Lratio of the second NMOS transistor circuit 514 may be realized by anumber of NMOS transistors connected in series. A gate of the secondNMOS transistor 516 is coupled to the first voltage source 506 and adrain of the second NMOS transistor 516 is coupled to a source-amplifierline 518 between a source and an amplifier device, such as thesource-amplifier line 106 connecting the source 102 and the amplifiercircuit 104 of FIG. 1. Additionally, the drain of the second NMOStransistor 516 is coupled to a first PMOS transistor circuit 520 and asource of the second NMOS transistor 516 is coupled to the line 508,thus providing a parallel connection between the second NMOS transistorcircuit 514 and the first PMOS transistor circuit 520. In someimplementations, the parallel connection between the second NMOStransistor circuit 514 and the first PMOS transistor circuit 520 mayproduce a clamping effect.

The first PMOS transistor circuit 520 may have a first PMOS channelwidth to channel length (W/L) ratio. In some implementations, the firstPMOS W/L ratio may be different from the second NMOS W/L ratio of thesecond NMOS transistor circuit 514. In addition, the first PMOStransistor circuit 520 includes one or more PMOS transistors representedby a first PMOS transistor 522. Thus, the W/L ratio of the first PMOStransistor circuit 520 may be realized in a single PMOS transistor or ina number of PMOS transistors. In some instances, the first PMOS W/Lratio may be realized by a number of PMOS transistors connected inseries. A source of the first PMOS transistor 522 is coupled to a secondvoltage source 524. In some implementations, a gate of the first PMOStransistor 522 is coupled to a reference, point, such as ground.

The second voltage source 524 includes a number of components, such as asecond impedance element 526 and a second current source 528. In someimplementations, the second voltage source 524 is also coupled to areference point, such as ground. Additionally, the second voltage source524 is coupled to a second PMOS transistor circuit 530. The second PMOStransistor circuit 530 may have a second PMOS W/L ratio that is greaterthan the first PMOS W/L ratio. The W/L ratio of the second PMOStransistor circuit 530 may be realized by one or more PMOS transistorsrepresented by a second PMOS transistor 532. In some instances, the W/Lratio of the second transistor circuit 530 may be realized by a numberof PMOS transistors connected in parallel. A source of the second PMOStransistor 532 is coupled to the second voltage source 524. In addition,the source of the second PMOS transistor 532 receives a second referencecurrent, I_(REF2). In some implementations, a drain of the second PMOStransistor 532 and a gate of the second PMOS transistor 532 are alsocoupled to ground. In some implementations, the second PMOS W/L ratiomay be different from the first NMOS W/L ratio of the first NMOStransistor circuit 502.

A first NMOS gate-source voltage of the first NMOS transistor 504 isdefined by the current I_(REF1) provided to the first NMOS transistor504 and the current drawn from the first NMOS transistor 504 by thefirst voltage source 506. In this way, a well-defined gate-sourcevoltage can be produced at the first NMOS transistor 504. Additionally,this arrangement may produce other well-defined small signal parameters,such as the transconductance of the first NMOS transistor 504. A secondNMOS gate-source voltage of the second NMOS transistor 516 is defined bya difference of the gate-source voltage of the first NMOS transistor 504and a voltage drop provided by the first voltage source 506. The voltagedrop of the first voltage source 506 may be defined by the product ofthe impedance value of the impedance element 510 and the current drawnby the first current source 512. Thus, the second NMOS transistor 516can have a well-defined gate-source voltage, which also results in awell-defined R_(ON), that is, the impedance value that turns on thesecond NMOS transistor 516.

In the illustrative implementation shown in FIG. 5, the first NMOStransistor circuit 502 and the second NMOS transistor circuit 514 areplaced in a current mirror type arrangement and the high impedancecircuit 500 may be used to generate an impedance transformation byproducing a first NMOS overdrive voltage at the first NMOS transistor504 and a second NMOS overdrive voltage at the second NMOS transistor516. In some implementations, the first NMOS overdrive voltage may bedifferent from the second NMOS overdrive voltage due to differinggate-source voltages at the first NMOS transistor 504 and the secondNMOS transistor 516, while the threshold voltages of the first NMOStransistor 504 and the second NMOS transistor 516 are approximately thesame. In other implementations, the first NMOS overdrive voltage may bedifferent from the second NMOS overdrive voltage due to differingthreshold voltages of the first NMOS transistor 504 and the second NMOStransistor 516, while the gate-source voltages of the first NMOStransistor 504 and the second NMOS transistor 516 are approximately thesame. In a particular example, when the second NMOS overdrive voltage isless than the first NMOS overdrive voltage, the impedance value of thefirst NMOS transistor 504 is transformed to the impedance value of thesecond NMOS transistor 516 giving a very high impedance value for thehigh impedance circuit 500.

In some instances, the gate-source voltage of the first NMOS transistor504, the gate-source voltage of the second NMOS transistor 516, or acombination thereof, may be in a sub-threshold region. When thegate-source voltage of the second NMOS transistor 516 is in thesub-threshold region, the impedance of the second NMOS transistor 516may increase exponentially as the overdrive voltage of the second NMOStransistor 516 decreases. The amount of increase in the impedance of thesecond NMOS transistor 516 may be based on the difference in theoverdrive voltages of the first NMOS transistor 504 and the second NMOStransistor 516.

When the NMOS W/L ratio of the second NMOS transistor circuit 514 isless than the NMOS W/L ratio of the first NMOS transistor circuit 502,the second NMOS transistor circuit 514 may serve as an impedance deviceand an impedance transformation may take place with respect to thesecond NMOS transistor circuit 514 that is proportional to the quotientof the NMOS W/L ratio of the first NMOS transistor circuit 502 and theNMOS W/L ratio of the second transistor circuit 514. Thus, by producinga first impedance transformation utilizing different W/L ratios for theNMOS transistor circuits 502, 514, in conjunction with producing asecond impedance transformation with different overdrive voltages forthe NMOS transistors 504, 516, the high impedance circuit 500 canprovide a very high input impedance for an amplifier device, such as theamplifier circuit 104 of FIG. 1.

A first PMOS gate-source voltage of the first PMOS transistor 522 isdefined by a difference of the gate-source voltage of the second PMOStransistor 532 and a voltage drop provided by the second voltage source524. The voltage drop of the second voltage source 524 may be defined bythe product of the impedance value of the second impedance element 526and the current drawn by the second current source 528. Thus, the firstPMOS transistor 522 can have a well-defined gate-source voltage, whichalso results in a well-defined R_(ON), that is, the impedance value thatturns on the first PMOS transistor 522. A second PMOS gate-sourcevoltage of the second PMOS transistor 532 is defined by the currentI_(REF2) provided to the second PMOS transistor 532 and the currentdrawn from the second PMOS transistor 532 by the second voltage source524. In this way, a well-defined gate-source voltage can be produced atthe second PMOS transistor 532. Additionally, this arrangement mayproduce other well-defined small signal parameters, such as thetransconductance of the second PMOS transistor 532.

In the illustrative implementation shown in FIG. 5, the first PMOStransistor circuit 520 and the second PMOS transistor circuit 530 areplaced in a current mirror type arrangement and the high impedancecircuit 500 may be used to generate an impedance transformation byproducing a first PMOS overdrive voltage at the first PMOS transistor522 and a second PMOS overdrive voltage at the second PMOS transistor532. In some implementations, the first PMOS overdrive voltage may bedifferent from the second PMOS overdrive voltage due to differinggate-source voltages at the first PMOS transistor 522 and the secondPMOS transistor 532, while the threshold voltages of the first PMOStransistor 522 and the second PMOS transistor 532 are approximately thesame. In other implementations, the first PMOS overdrive voltage may bedifferent from the second PMOS overdrive voltage due to differingthreshold voltages of the first PMOS transistor 522 and the second PMOStransistor 532, while the gate-source voltages of the first PMOStransistor 522 and the second PMOS transistor 532 are approximately thesame. In a particular example, when the first PMOS overdrive voltage isless than the second PMOS overdrive voltage, the impedance value of thesecond PMOS transistor 532 is transformed to the impedance value of thefirst PMOS transistor 522 giving a very high impedance value for thehigh impedance circuit 500.

In some instances, the gate-source voltage of the first PMOS transistor522, the gate-source voltage of the second PMOS transistor 532, or acombination thereof, may be in a sub-threshold region. When thegate-source voltage of the first PMOS transistor 522 is in thesub-threshold region, the impedance of the first PMOS transistor 522 mayincrease exponentially as the overdrive voltage of the first PMOStransistor 522 decreases. The amount of increase in the impedance of thefirst PMOS transistor 522 may be based on the difference in theoverdrive voltages of the first PMOS transistor 522 and the second PMOStransistor 532.

When the W/L ratio of the second PMOS transistor circuit 530 is greaterthan the W/L ratio of the first transistor circuit 520, the first PMOStransistor circuit 520 may serve as an impedance device and an impedancetransformation may take place with respect to the first PMOS transistorcircuit 520 that is proportional to the quotient of the W/L ratio of thefirst PMOS transistor circuit 520 and the W/L ratio of the second PMOStransistor circuit 530. Thus, by producing a third impedancetransformation utilizing different W/L ratios for the first PMOStransistor circuit 520 and the second PMOS transistor circuit 530, inconjunction with producing a fourth impedance transformation withdifferent overdrive voltages for the PMOS transistors 522, 532,contributes to the impedance provided by the high impedance circuit 500to the amplifier device.

In some instances, impedance transformations via the first NMOStransistor circuit 502 and the second NMOS transistor circuit 514 mayproduce non-linear behavior with respect to the impedance provided bythe high impedance circuit 500. The impedance transformations that occurwith respect to the first PMOS transistor circuit 520 and the secondPMOS transistor circuit 530 helps to offset at least some of thenon-linearity in the impedance produced by the first NMOS transistorcircuit 502 and the second NMOS transistor circuit 514.

FIG. 6 is a schematic diagram of a high impedance circuit 600 includingNMOS transistor arrangements coupled to an impedance element and acurrent source to provide different overdrive voltages to the NMOStransistor arrangements. The high impedance circuit 600 includes a firsttransistor circuit 602. The first transistor circuit 602 may have afirst channel width to channel length (W/L) ratio. In addition, thefirst transistor circuit 602 includes one or more NMOS transistorsrepresented by a first NMOS transistor 604. Thus, the W/L ratio of thefirst transistor circuit 602 may be realized in a single NMOS transistoror in a number of NMOS transistors. In some instances, the W/L ratio maybe realized by a number of NMOS transistors connected in parallel. Agate of the first NMOS transistor 604 is coupled to a first impedanceelement 606 and a source of the first NMOS transistor 604 is coupled toground. A drain of the first NMOS transistor 604 is coupled to a secondimpedance element 608. The second impedance element 608 receives areference current, I_(REF1).

The first impedance element 606 is coupled to a second transistorcircuit 610 and a current source 612. The second transistor circuit 610may have a second W/L ratio that is less than the first W/L ratio. TheW/L ratio of the second transistor circuit 610 may be realized by one ormore NMOS transistors represented by a second NMOS transistor 614. Insome instances, the W/L ratio of the second transistor circuit 610 maybe realized by a number of NMOS transistors connected in series. A gateof the second NMOS transistor 614 is coupled to the first impedanceelement 606 and to the current source 612. In some implementations, asource of the second NMOS transistor 614 is coupled to a referencepoint, such as ground. Further, a drain of the second NMOS transistor614 is coupled to a source-amplifier line 616. The source-amplifier line616 is coupled between a source and an amplifier device, such as thesource-amplifier line 106 connecting the source 102 and the amplifiercircuit 104 of FIG. 1.

The current source 612 includes a third NMOS transistor 618 in a currentmirror arrangement with a fourth NMOS transistor 620. A drain of thethird NMOS transistor 618 is coupled to the first impedance element 606and the second transistor circuit 610. In some implementations, a sourceof the third NMOS transistor 618 is coupled to a reference point, suchas ground. In addition, a gate of the third NMOS transistor 618 iscoupled to a gate of the fourth NMOS transistor 620. A source of thefourth NMOS transistor 620 may be coupled to ground and a drain of thefourth NMOS transistor 620 is coupled to a third impedance element 622.The third impedance element 622 receives a reference current, I_(REF2).

A first NMOS gate-source voltage of the first NMOS transistor 604 isdefined by the current I_(REF1) provided to the first NMOS transistor604 and the current drawn from the first NMOS transistor 604 by thecurrent source 612. In this way, a well-defined gate-source voltage canbe produced at the first NMOS transistor 604. A second NMOS gate-sourcevoltage of the second NMOS transistor 614 is defined by a difference ofthe gate-source voltage of the first NMOS transistor 604 and a voltagedrop defined by the product of the impedance value of the impedanceelement 606 and the current drawn by the current source 612. Thus, thesecond NMOS transistor 614 can have a well-defined gate-source voltage,which also results in a well-defined R_(ON), that is, the impedancevalue that turns on the second NMOS transistor 614.

In the illustrative implementation shown in FIG. 6, the first transistorcircuit 602 and the second transistor circuit 610 are placed in acurrent mirror type arrangement and the high impedance circuit 600 maybe used to generate an impedance transformation by producing a firstoverdrive voltage at the first NMOS transistor 604 and a secondoverdrive voltage at the second NMOS transistor 614 that is differentfrom the first overdrive voltage. In some implementations, an overdrivevoltage of the first NMOS transistor 604 may be different from anoverdrive voltage of the second NMOS transistor 614 due to differinggate-source voltages at the first NMOS transistor 604 and the secondNMOS transistor 614, while the threshold voltages of the first NMOStransistor 604 and the second NMOS transistor 614 are approximately thesame. In other implementations, the overdrive voltage of the first NMOStransistor 604 may be different from the overdrive voltage of the secondNMOS transistor 614 due to differing threshold voltages of the firstNMOS transistor 604 and the second NMOS transistor 614, while thegate-source voltages of the first NMOS transistor 604 and the secondNMOS transistor 614 are approximately the same. In a particular example,when the overdrive voltage of the second NMOS transistor 614 is lessthan the overdrive voltage of the first NMOS transistor 604, theimpedance value of the first NMOS transistor 604 is transformed to theimpedance value of the second NMOS transistor 614 giving a very highimpedance value for the high impedance circuit 600.

In some instances, the gate-source voltage of the first NMOS transistor604, the gate-source voltage of the second NMOS transistor 614, or acombination thereof, may be in a sub-threshold region. When thegate-source voltage of the second NMOS transistor 614 is in thesub-threshold region, the impedance of the second transistor circuit 610may increase exponentially as the overdrive voltage of the second NMOStransistor 614 decreases. The amount of increase in the impedance of thesecond NMOS transistor 614 may be based on the difference in theoverdrive voltages of the first NMOS transistor 604 and the second NMOStransistor 614.

When the W/L ratio of the second transistor circuit 610 is less than theW/L ratio of the first transistor circuit 602, the second transistorcircuit 610 may serve as an impedance device and an impedancetransformation may take place with respect to the second transistorcircuit 610 that is proportional to the quotient of the first W/L ratioand the second W/L ratio. Thus, by producing a first impedancetransformation by different overdrive voltages for the NMOS transistors604, 614, in conjunction with a second impedance transformationutilizing different W/L ratios for the transistor circuits 602, 610, thehigh impedance circuit 600 can provide a very high impedance for anamplifier device, such as the amplifier circuit 104 of FIG. 1.

In addition, in some instances, the current source 612 may depart fromideal behavior due to second-order effects, such as the Early effect. Tooffset the second-order effects with respect to the current source 612,the drain source voltages of the first NMOS transistor 604, the thirdNMOS transistor 618, and the fourth NMOS transistor 620 may beapproximately the same. In this way, the current produced by the currentsource 612 can be relatively stable and well-defined. In someimplementations, the current source 612 may be used as the currentsource 210 of FIG. 2, the first current source 412 and the secondcurrent source 428 of FIG. 4, and the first current source 512 and thesecond current source 528 of FIG. 5. A current source similar to that ofthe current source 612 using PMOS transistors connected to V_(DD) ratherthan ground may be used as the current source 310 of FIG. 3. Further, anadditional impedance element, such as the second impedance element 608,may be coupled to the drain of the first NMOS transistor 204 of FIG. 2,a drain of the first PMOS transistor 304 of FIG. 3, a drain of the firstNMOS transistor 404 of FIG. 4, and a drain of the first NMOS transistor504 of FIG. 5.

FIG. 7 is a schematic diagram of a high impedance circuit 700 includinga gate to drain coupling of NMOS transistor circuits having differentoverdrive voltages. The high impedance circuit 700 includes a firsttransistor circuit 702. The first transistor circuit 702 may have afirst channel width to channel length (W/L) ratio. In addition, thefirst transistor circuit 702 includes one or more NMOS transistorsrepresented by a first NMOS transistor 704. Thus, the W/L ratio of thefirst transistor circuit 702 may be realized in a single NMOS transistoror in a number of NMOS transistors. In some instances, the W/L ratio maybe realized by a number of NMOS transistors connected in parallel. Asource of the first NMOS transistor 704 may be coupled to ground and adrain of the first NMOS transistor 704 is coupled to an impedanceelement 706. The impedance element 706 receives a reference current,I_(REF). Further, a gate of the first NMOS transistor 704 also receivesthe reference current, I_(REF).

The high impedance circuit 700 also includes a second transistor circuit708. In the illustrative implementation shown in FIG. 7, the firsttransistor circuit 702 and the second transistor circuit 708 are placedin a current mirror type arrangement. The second transistor circuit 708may have a second W/L ratio that is less than the first W/L ratio. TheW/L ratio of the second transistor circuit 708 may be realized by one ormore NMOS transistors represented by a second NMOS transistor 710. Insome instances, the W/L ratio of the second transistor circuit 708 maybe realized by a number of NMOS transistors connected in series. A gateof the second NMOS transistor 710 is coupled to the drain of the firstNMOS transistor 704 and a source of the second NMOS transistor 710 maybe coupled to ground. Further, a drain of the second NMOS transistor 710is coupled to a source-amplifier line 712. The source-amplifier line 712is coupled between a source and an amplifier device, such as thesource-amplifier line 106 connecting the source 102 and the amplifiercircuit 104 of FIG. 1.

A gate source voltage of the first NMOS transistor 704 is defined by thecurrent, I_(REF), and the gate-source voltage of the second NMOStransistor 710 is defined by a difference of the gate-source voltage ofthe first NMOS transistor 704 and a voltage defined by the product ofthe impedance value of the impedance element 706 and the value of thecurrent I_(REF). In some implementations, the gate-source voltage of thefirst NMOS transistor 704 may be in the sub-threshold region, thegate-source voltage of the second NMOS transistor 710 may be in thesub-threshold region, or a combination thereof.

An impedance transformation may occur by producing different overdrivevoltages of the first NMOS transistor 704 and the second NMOS transistor710. In some implementations, an overdrive voltage of the first NMOStransistor 704 may be different from an overdrive voltage of the secondNMOS transistor 710 due to differing gate-source voltages at the firstNMOS transistor 704 and the second NMOS transistor 710, while thethreshold voltages of the first NMOS transistor 704 and the second NMOStransistor 710 are approximately the same. In these implementations, thedifference between the gate-source voltage of the first NMOS transistor704 and the gate-source voltage of the second NMOS transistor 710 is dueto the impedance element 706 coupled to the drain of the second NMOStransistor 710. In other implementations, the overdrive voltage of thefirst NMOS transistor 704 may be different from the overdrive voltage ofthe second NMOS transistor 710 due to differing threshold voltages ofthe first NMOS transistor 704 and the second NMOS transistor 710, whilethe gate-source voltages of the first NMOS transistor 704 and the secondNMOS transistor are approximately the same. In a particular example,when the overdrive voltage of the second NMOS transistor 710 is lessthan the overdrive voltage of the first NMOS transistor 704, theimpedance value of the first NMOS transistor 704 is transformed to theimpedance value of the second NMOS transistor 710 giving a very highimpedance value for the high impedance circuit 700. When the gate-sourcevoltage of the second NMOS transistor 710 is in the sub-thresholdregion, the impedance of the second NMOS transistor 710 may increaseexponentially as the overdrive voltage of the second NMOS transistor 710decreases. The amount of increase in the impedance of the second NMOStransistor 710 may be based on the difference in the overdrive voltagesof the first NMOS transistor 704 and the second NMOS transistor 710.

Further, when the W/L ratio of the second transistor circuit 708 is lessthan the W/L ratio of the first transistor circuit 702, the secondtransistor circuit 708 may serve as an impedance device and an impedancetransformation may take place with respect to the second transistorcircuit 708 that is proportional to the quotient of the W/L ratio of thefirst transistor circuit 702 and the W/L ratio of the second transistorcircuit 708. Thus, by producing an impedance transformation due todiffering overdrive voltages of the first NMOS transistor 704 and thesecond NMOS transistor 710 and by producing an impedance transformationdue to differing W/L ratios of the first transistor circuit 702 and thesecond transistor circuit 708, the high impedance circuit 700 canprovide very high input impedance for an amplifier device, such as theamplifier circuit 104 of FIG. 1.

FIG. 8 is a flow diagram of a method 800 of producing impedancetransformations in a high impedance circuit, such as the high impedancecircuits 110, 200, 300, 400, 500, 600, and 700 of FIGS. 1-7,respectively.

Specifics of exemplary methods are described below. However, it shouldbe understood that certain acts need not be performed in the orderdescribed, and may be modified, and/or may be omitted entirely,depending on the circumstances. Moreover, the acts described may beimplemented by a computer, processor or other computing device based oninstructions stored on one or more computer-readable storage media. Thecomputer-readable storage media can be any available media that can beaccessed by a computing device to implement the instructions storedthereon.

At 802, a first NMOS overdrive voltage is produced for a first NMOStransistor circuit. The first NMOS overdrive voltage is defined by thevalue of the threshold voltage of the first NMOS transistor circuitsubtracted from the gate-source voltage of the first NMOS transistorcircuit. The gate-source voltage of the first NMOS transistor circuitmay by produced by applying a first bias current to the first NMOStransistor circuit. The gate-source voltage of the first NMOS transistormay also be in the sub-threshold region of the first NMOS transistorcircuit. In addition, the first NMOS transistor circuit may have a firstNMOS W/L ratio that is realized by a number of NMOS transistors coupledin parallel.

At 804, a second NMOS overdrive voltage is produced for a second NMOStransistor circuit that is different from the first NMOS overdrivevoltage. The second NMOS overdrive voltage is defined by the value ofthe threshold voltage of the second NMOS transistor circuit subtractedfrom the gate-source voltage of the second NMOS transistor circuit. Thegate-source voltage of the second NMOS transistor circuit may be definedby modifying the gate-source voltage of the first NMOS transistorcircuit using a first voltage source. The gate-source voltage of thesecond NMOS transistor circuit may also be in the sub-threshold regionof the second NMOS transistor circuit. In addition, the second NMOStransistor circuit may have a second NMOS W/L ratio that is realized bya number of NMOS transistors coupled in series. In some implementations,the second NMOS overdrive voltage is less than the first NMOS overdrivevoltage. In this way, the second NMOS transistor circuit acts as animpedance device and an impedance transformation takes place due to thediffering overdrive voltages. An additional impedance transformation mayalso take place due to the differing W/L ratios of the first NMOStransistor circuit and the second NMOS transistor circuit.

At 806, a first PMOS overdrive voltage is produced for a first PMOStransistor circuit. The first PMOS overdrive voltage is defined by thevalue of the threshold voltage of the first PMOS transistor circuitsubtracted from the gate-source voltage of the first PMOS transistorcircuit. The gate-source voltage of the first PMOS transistor circuitmay by produced by applying a second bias current to the first PMOStransistor circuit. The gate-source voltage of the first PMOS transistorcircuit may also be in the sub-threshold region of the first PMOStransistor circuit. In addition, the first PMOS transistor circuit mayhave a first PMOS W/L ratio that is realized by a number of PMOStransistors coupled in parallel.

At 808, a second PMOS overdrive voltage is produced for a second PMOStransistor circuit that is different from the first PMOS overdrivevoltage. The second PMOS overdrive voltage is defined by the value ofthe threshold voltage of the second PMOS transistor circuit subtractedfrom the gate-source voltage of the second PMOS transistor circuit. Thegate-source voltage of the second PMOS transistor circuit may be definedby modifying the gate-source voltage of the first PMOS transistorcircuit using a second voltage source. The gate-source voltage of thesecond PMOS transistor circuit may also be in the sub-threshold regionof the second PMOS transistor circuit. In addition, the second PMOStransistor circuit may have a second PMOS W/L ratio that is realized bya number of PMOS transistors coupled in series. Further, the second PMOStransistor circuit may be coupled in series or parallel with the secondNMOS transistor circuit. In some implementations, the second PMOSoverdrive voltage is less than the first PMOS overdrive voltage. In thisway, the second PMOS transistor circuit acts as an impedance device andan impedance transformation takes place due to the differing overdrivevoltages. An additional impedance transformation may also take place dueto differing W/L ratios of the first PMOS transistor circuit and thesecond PMOS transistor circuit. Further, by producing impedancetransformations utilizing a pair of NMOS transistor circuits andproducing impedance transformations utilizing a pair of PMOS transistorcircuits in a high impedance circuit, non-linear behavior of the highimpedance circuit may be reduced.

CONCLUSION

For the purposes of this disclosure and the claims that follow, theterms “coupled” and “connected” have been used to describe how variouselements interface. Such described interfacing of various elements maybe either direct or indirect. Although the subject matter has beendescribed in language specific to structural features and/ormethodological acts, it is to be understood that the subject matterdefined in the appended claims is not necessarily limited to thespecific features or acts described. Rather, the specific features andacts are disclosed as preferred forms of implementing the claims. Thespecific features and acts described in this disclosure and variationsof these specific features and acts may be implemented separately or maybe combined.

1. An apparatus comprising: a first transistor circuit having a firstoverdrive voltage, wherein the first overdrive voltage is defined by agate-source voltage of the first transistor circuit minus a thresholdvoltage of the first transistor circuit; and a second transistor circuithaving a second overdrive voltage, wherein the second overdrive voltageis defined by a gate-source voltage of the second transistor circuitminus a threshold voltage of the second transistor circuit, and whereinthe second transistor circuit is placed in a current mirror typearrangement with the first transistor circuit, wherein an impedance ofthe second transistor circuit increases as the second overdrive voltagedecreases with respect to the first overdrive voltage and wherein thethreshold voltage of the second transistor circuit is higher than thethreshold voltage of the first transistor circuit and the gate-sourcevoltage of the second transistor circuit is approximately equal to theqate-source voltage of the first transistor circuit.
 2. The apparatus ofclaim 1, wherein the first transistor circuit has a first channel widthto channel length ratio and the second transistor circuit has a secondchannel width to channel length ratio that is smaller than the firstchannel width to channel length ratio.
 3. The apparatus of claim 1,wherein the first gate-source voltage, the second gate-source voltage,or a combination thereof, are in a sub-threshold region.
 4. Theapparatus of claim 1, further comprising a voltage source coupled to thefirst transistor circuit and the second transistor circuit, wherein thevoltage source includes a first impedance element and a current source.5. The apparatus of claim 4, wherein a gate of the first transistorcircuit is coupled to the first impedance element and a gate of thesecond transistor circuit is coupled to the first impedance element. 6.The apparatus of claim 4, wherein the current source comprises a currentmirror arrangement including a first NMOS transistor and a second NMOStransistor.
 7. The apparatus of claim 1, wherein the first transistorcircuit includes a plurality of transistors connected in parallel. 8.The apparatus of claim 1, wherein the second transistor circuit includesa plurality of transistors connected in series.
 9. An apparatuscomprising: a first negative channel metal oxide semiconductor (NMOS)transistor circuit having a first channel width to channel length ratio;a second NMOS transistor circuit having a second channel width tochannel length ratio, wherein a gate of the second NMOS transistorcircuit is coupled to a drain of the first NMOS transistor circuit, andwherein the second channel width to channel length ratio is less thanthe first channel width to channel length ratio; and an impedanceelement coupled to the drain of the first NMOS transistor circuit,wherein a drain of the second NMOS transistor circuit is coupled to asource-amplifier line connecting a source and an amplifier circuit.